1. Field of the Invention
The present invention relates to methods of distributing document data containing such data as image data and music data, apparatus for creating the document data, apparatus for accessing the document data, and programs thereof. The invention also relates to methods of managing the documents, apparatus for managing the documents, and programs thereof.
2. Description of Related Art
Electronic signature is known as a technique that can certify authenticity of document data containing digitized data such as image data and music data.
For example, a hash value of digitized data is computed and is then encrypted using a private key to produce an electronic signature, and the digitized data are distributed with the electronic signature appended thereto. A user who receives the distributed digitized data decrypts the electronic signature using a public key encryption and at the same time computes a hash value of the digitized data, and by verifying if they match or not, the user can authenticate the received digitized data.
With this technique, unauthorized alteration to distributed document data can be detected, and unauthorized data distribution can thus be prevented.
The encrypted data that are appended as an electronic signature are, however, visible data, and there is a possibility that they may be deciphered if a plurality of such data are analyzed through algorithmic analysis.
Another technique has been suggested in which an encryption key or the like is embedded as an invisible electronic watermark into image data that constitute the document data. However, in order to extract the electronic watermark from the document data to perform access control, a device equipped with a special application program is necessary.
The present invention proposes a document distribution method that achieves combined functions of concealing or hiding of, preventing unauthorized alteration of, authenticating of, and preventing unauthorized access to document data by using access control information that is separated into invisible data that are embedded as an electronic watermark in digitized data and visible data that are described in a structured text.